Add executor_setup and executor_teardown config sections

[drobertduke]

We need sections for setting up / tearing down per executor.

[josephharrington]

@drobertduke @tjlee0909 Do you guys remember what the use case for this was?

[drobertduke]

The use case we had was browser-based tests. For browser-based tests we need to start selenium, start a browser, load the seed database into mysql and wire everything up. These resources are too heavy to start and tear down per-atom, and they can't be managed per-machine because they don't support parallelization (you don't want multiple executors hitting the same browser). A per-executor setup would allow the management of resources that require serialized tests, but are too heavy to start/stop per atom.